The present invention is directed to processes for forming styrenic copolymers, and their related end uses. In particular, the present invention is directed to processes for preparing patterns for use in metal castings.
Styrenic polymers have a wide variety of applications, including the formation of expanded polystyrene which can be used to make a variety of products. Processes for forming styrenic polymers include emulsion polymerization, suspension polymerization, and the use of particular suspensions or emulsion aids.
Polymer particles are useful in applications such as the formation of expanded resins, for example, expanded polystyrene. Expanded polystyrene and other expanded resins can be prepared from expandable polymeric particles made by contacting the polymeric particles with a volatile compound known as a xe2x80x9cblowing agentxe2x80x9d or xe2x80x9cexpanding agentxe2x80x9d. Such agents include aliphatic hydrocarbons such as butane, pentanes, hexanes, and halogenated hydrocarbons such as trichloromethane, trichlorofluoromethane, and methyl chloride. The particles in contact with the expanding agent may be expanded by heating, or by exposure to reduced pressure as in a vacuum. The size and size distribution of the expanded particles will depend upon the size and size distribution of the expandable beads.
Expanded and expandable polymeric resins have applications in packaging, consumer products, and in materials processing. Examples of materials processing applications for expanded polymeric resins include so-called xe2x80x9clost foam castingxe2x80x9d, also called xe2x80x9cevaporative pattern castingxe2x80x9d. In lost foam casting, molten metal is poured into a pattern made of expanded polymeric material, i.e. a foam, coated with a refractory material surrounded and supported by unbounded sand. The foam is decomposed by the heat of the molten metal and replaced by the metal.
A need remains for new and/or improved processes for forming styrenic polymers, as well as for related improvements in lost foam casting applications.
The present invention is related to a process for the preparation of a vinyl aromatic polymer, e.g., a polystyrene suitable for lost foam casting applications. Pre-expanded beads (prepuff) prepared from polystyrene containing an effective amount of a combination of a bromine-attached aliphatic or aromatic flame retardant and optionally dicumyl peroxide can be used in conventional steam molding equipment to produce low density patterns. Aluminum castings made from the polystyrene/combination material show significantly less signs of carbon deposits, although any metal may be benefited by the technology of the present invention. The polystyrene smoothly and controllably decomposes to give a smooth, clean metal casting.
The vinyl aromatic polymer particles suitable for use in the process of this invention may be spherical or irregularly shaped particles of any of the thermoplastic vinyl aromatic polymers usable in the preparation of molded foam articles. Although homopolymers or copolymers of any vinyl aromatic monomer may be employed, styrene and substituted styrene monomers are preferred. Examples of suitable vinyl aromatic monomers include, but are not limited to, styrene, xcex1-methyl styrene, aryl-methyl styrene, aryl-ethyl styrene, aryl-isopropyl styrene, aryl-tert-butyl styrene, vinyl toluene, vinyl xylene, aryl-chlorostyrene, aryl-chloromethylstyrene, vinyl napthalene, divinyl benzene, and the like. Minor amounts (i.e., up to about 50 mole percent) of other ethylenically unsaturated copolymerizable monomers may also be used, including, for example, butadiene, acrylic acid, methacrylic acid, maleic anhydride, methyl methacrylate, acrylonitrile, and the like. The vinyl aromatic polymer may be rubber modified with an elastomer such as polybutadiene or styrene/butadiene block or random copolymers. The vinyl aromatic polymer particles should preferably be from about 0.1 to 2 mm in average diameter. Methods of obtaining suitable particles such as suspension polymerization or pelletization are well known in the art.
The polymers useful in the present invention include polystyrene having a molecular weight of 150,000 to 350,000, preferably from about 170,000 to 320,000. Small spherical beads of polymer having bead diameters between 100 and 600 microns, preferably between 150-500 microns, and most preferably between 250-425 microns are useful for purposes of the present invention.
Thus, the present invention is directed to a process for preparing a pattern for use in making metal castings (e.g., aluminum, brass, bronze, ductile, modular or grey iron, magnesium or steel) which have significantly less residual carbon on the surface which comprises:
(a) adding an amount, effective for the purpose, of a combination of a bromine-attached aliphatic or aromatic flame retardant and optionally dicumyl peroxide to a suspension of vinyl aromatic polymer particles having a molecular weight of about 150,000 to 350,000 and having a bead size between 100 and 600 microns in diameter; and
(b) adding a suitable blowing agent to the beads and heating to impregnate the beads.
By bromine-attached aliphatic or aromatic flame retardant, it is meant an organic bromine compound having more than 40% by weight bromine and not more than 80% by weight bromine. From about 0.20 to 1.2 parts by weight of flame retardant per 100 parts by weight of vinyl aromatic polymer particles is needed to be effective. Optionally, from about 0.01 to 0.20 percent of the dicumyl peroxide material is added to the system in need of treatment. However, it can be envisioned that a range of up to 5.0 wt. % flame retardants may be required in certain instances to reduce the carbon defects to an insignificant amount.
Suitable blowing agents are, e.g., butane, n-pentane, isopentane, cyclopentane, hexanes, cyclohexane, carbon dioxide, fluorinated hydrocarbons and mixtures thereof. The combination of the bromine-attached aliphatic or aromatic flame retardants and optionally dicumyl peroxide may be added to the suspension as well as the blowing agent.
A number of brominated fire retardant materials are effective for purposes of the present invention. The HBCD to be used as the fire-retardant agent in the process of this invention can be any of the hexabrominated derivatives of cyclododecatriene. Any of the isomers of hexabromocyclododecane are suitable for use. Mixtures of different isomers of hexabromocyclododecane can also be employed. The average particle size of the hexabromocyclododecane may be less than about 100 microns, and is preferably less than about 25 microns. HBCD is available commercially from Ameribrom, Inc., Albermarle Corp. (xe2x80x9cSAYTEX HBCDxe2x80x9d), and Great Lakes Chemical Corp. (xe2x80x9cCD-75Pxe2x80x9d).
The fire-retardant expandable vinyl aromatic polymer beads produced by the process of this invention may be readily shaped into molded foam articles by heating in molds which are not gastight when closed. The beads expand to form prepuff which after aging can be steamed and fused together to form the molded article. Such methods of preparing molded-bead foams are well-known and are described, for example, in Ingram et al, xe2x80x9cPolystyrene and Related Thermoplastic Foamsxe2x80x9d Plastic Foams, Marcel Dekker (1973), Part II, Chapter 10, pp.531-581, Ingram xe2x80x9cExpandable Polystyrene Processesxe2x80x9d Addition and Condensation Polymerization Process American Chemical Society (1969), Chapter 33, pp. 531-535.
Molded foam articles prepared using the fire-retardant expandable vinyl aromatic beads of this invention are resistant to flame, even when relatively low levels of the flame retardant (e.g., hexabromocyclododecane and others) are present. The hexabromocyclododecane is incorporated with the beads rather than coated on the surface of the beads and thus does not interfere with the fusion of the beads when they are expanded into molded foam articles. The density, tensile strength, heat resistance and other physical and mechanical properties of the foams are unaffected by the presence of the hexabromocyclododecane if the process of this invention is employed.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following examples, therefore, are to be considered as merely illustrative and not limitative of the claims or remainder of the disclosure in any way whatsoever.